The prior art includes one-piece, T-shaped blanks for forming cartons to hold liquids, such as beverages. One such prior art patent application was filed by Mitchell J. Matovich, Jr., on Nov. 5, 1976 and bears Ser. No. 739,368 now U.S. Pat. No. 4,084,489. The T-shape in such prior blanks results from the provision of end closure panels at the opposite ends of one of the side wall panels of the carton. Each of the remaining side wall panels is provided at its ends with sealing tabs to be bent inwardly after the blank has been folded into the configuration of an open ended tube, and side seamed by means of a side seam flap carried by the same side wall panel with which the two end closure panels are associated. The free edges of the end closure panels are designed to be sealed to the sealing tabs on the ends of the side wall panels and, in the cited prior application as well as most prior art used to form flat-ended cartons, the edges of the end closure panels, or at least the last of the end closure panels to be closed, and the sealing flaps on the side wall panels adjacent that end, are first heated to render the polyethylene or other material sealing the carton in a semi-molten condition. After such heating, the end closure panel is pressed onto the top of the previously formed carton side walls to close and seal the carton over its contained liquid.
The prior art also recognizes the fact that there are substantial advantages to utilizing ultrasonic energy as a means of heating and sealing cartons. Unfortunately, ultrasonic heating requires that the pair of carton edges being sealed to one another be placed under pressure between an ultrasonic horn and an anvil, these two members pressing toward one another during the application of ultrasonic energy.
Ultrasonic sealing involves the vibration of an ultrasonic horn relative to a stationary anvil which must be heavy enough to remain stationary inspite of the vibration of the ultrasonic horn against it. The marginal edges of the paper carton which are placed between the ultrasonic horn and the anvil are heated in the regions where they are hammered between the horn and the anvil. The heating of these edges is dependent upon the pressure applied by the rapid hammering of the ultrasonic horn against the paperboard carton lying on the anvil. If the anvil is too light in weight, the anvil will vibrate with the ultrasonic horn, and very little relative motion will occur between the ultrasonic horn and the anvil. Therefore, very little heat will be induced in the paperboard carton. Furthermore, the anvil must provide a continuous area of support for the marginal edges beneath the ultrasonic horn in order for a continuous seal to be effected. In areas where the support provided by the anvil to the marginal edges is discontinuous, no heating will occur in the marginal edges, and therefore no seal will result in those areas. A further requirement for effecting a continuous seal is that the paperboard carton itself be continuous in the regions where a seal is to be effected. Thus, gaps or tears in the marginal edges will interrupt the formation of heat during the pounding of these edges between the ultrasonic horn and the anvil.
When sealing the side walls of a previously beverage-filled carton to the top end closure panel, it is impossible to place a sealing anvil inside of the container. Thus, no adequate means has been discovered to permit sealing such a carton utilizing ultrasonic energy.
It is believed to be known to form the upper carton end (after beverage filling) into a gable roof which permits a single gable roof seam to be sealed ultrasonically while being pinched between an ultrasonic horn and anvil, each of which lies in a plane which is perpendicular to a plane defining the flat upper end of the carton. While such gable roofed cartons operate satisfactorily, they do not facilitate stacking on top of one another in a larger carton, as is desirable when the beverage cartons are relatively small (as with one-half and one-third quart milk cartons). In addition, the formation of the gable top on these cartons requires a substantial increase in the amount of carton material required for the beverage container since typically the entire gable top is void of beverage and is simply extra paper stock material folded to provide the gable roof.